Media path issue in extensions/lib.py

If you had moved your MEDIA_ROOT somewhere else, this wouldn't have
worked probably.

<pre>
Traceback (most recent call last):

  File "/srv/modoboa/.env/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/srv/modoboa/.env/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 25, in _wrapped_view
    return view_func(request, *args, **kwargs)

  File "/srv/modoboa/.env/local/lib/python2.7/site-packages/modoboa-0.9.5-py2.7.egg/modoboa/lib/decorators.py", line 19, in wrapped_f
    return f(request, *args, **kwargs)

  File "/srv/modoboa/.env/local/lib/python2.7/site-packages/modoboa-0.9.5-py2.7.egg/modoboa/extensions/webmail/views.py", line 416, in getmailcontent
    email = ImapEmail(mbox, mailid, request, links=int(request.GET["links"]))

  File "/srv/modoboa/.env/local/lib/python2.7/site-packages/modoboa-0.9.5-py2.7.egg/modoboa/extensions/webmail/lib.py", line 226, in __init__
    self._fetch_inlines()

  File "/srv/modoboa/.env/local/lib/python2.7/site-packages/modoboa-0.9.5-py2.7.egg/modoboa/extensions/webmail/lib.py", line 301, in _fetch_inlines
    fp = open(path, "wb")

IOError: [Errno 2] No such file or directory: u'/srv/modoboa/modomail/modomail/media/webmail/974_pic3494219'
</pre>